Low Impact Development (LID) is a design strategy that aims to minimize the impact of development on the local watershed. It works by maintaining and replicating the natural hydrologic patterns that the landscape had prior to development.

Conventional approaches to design and development typically create large areas that are impervious to stormwater. In other words, large areas like buildings, parking lots, and streets replace or cover the natural soils and vegetation and allow water to run over them at a very fast rate. (For more information on the dangers of impervious surfaces and their effect on water quality and flooding, see Impervious Surfaces) Further, conventional approaches typically convey this runoff water to large facilities off site from the property. There, pollutants are sometimes removed before the water is released into a water body. This method negatively impacts the watershed and the ecosystems that depend on it even when pollutants are removed. Conventional approaches lead to:

“Flashy” hydrology – when streams are loaded with a high volume of water over a short span of time

Changes in the natural temperature of water bodies, and

Prevention of groundwater to recharge.

LID practices use several integrated design techniques to mimic the natural hydrology of a particular landscape.

Micro-scale stormwater retention and detention areas

Reduction of impervious surfaces

Lengthening of flow paths and runoff time

Preservation of environmentally sensitive areas and site features – such as riparian buffers, wetlands, steep slopes, mature trees, floodplains, woodlands, and highly permeable soils

These solutions can be integrated into the infrastructure of a development and have the added bonus of being more cost effective and aesthetically pleasing than traditional, stormwater conveyance systems. Designs can save a developer well over 50% of stormwater constructions costs.

Typical Components of a LID Design

Top

Vegetation (plants) functions in the removal of water through evapotranspiration and pollutant removal through nutrient cycling.
Vegetated Roof Covers have been extensively used and studied in Europe (Germany in particular) and are especially effective in older urban areas with chronic combined sewer overflow problems due to a high level of impervious surfaces. ”Green“ roofs reduce energy costs, lower the heat island effect, enhance air quality, and conserve valuable land that would otherwise be required for stormwater runoff controls.Disconnecting rain gutters from the storm sewer lines and redirecting the water into grass swales, bioretention systems, and other functional landscape devices. This water may also be saved in cisterns or rain barrels for later use in irrigation.

Permeable pavements or porous pavements are best suited for low traffic areas such as sidewalks and parking lots. These pavements allow stormwater to infiltrate into the underlying soils thereby promoting recharge and the treatment of pollution as well as reducing the volume of rainfall runoff produced by impermeable sidewalks and parking lots.

Bioretention systems are used to detain runoff water long enough for infiltration and pollution removal to occur. Their design is based upon local soil types, site conditions, and land uses and is quite cost effective. They typically include the following components:

Grass Buffer Strips reduce runoff velocity and filter particulate matter

Sand bed provides aeration and drainage of the planting soil and assists in the flushing of pollutants from soil materials.

Ponding area provides storage of excess runoff and facilitates the settling of particulates and evaporation of excess water.

Organic layer performs the function of decomposition of organic material by providing a medium for biological growth (such as microorganisms) to degrade petroleum-based pollutants. It also filters pollutants and prevents soil erosion.

Planting soil provides the area for stormwater storage and nutrient uptake by plants. The planting soils contain some clays that adsorb pollutants such as hydrocarbons, heavy metals, and nutrients.

Grass Swales are primarily used along residential streets and highways. They reduce runoff velocity and act as filtration/infiltration devices. Grass channels are most effective when the flow depth is minimized and the detention time is maximized.

Benefits of LID

Top

Provide a high level of water quality treatment controls due to runoff volume control of the “first flush” (first ½ inch) of runoff, which has the highest load of pollution. Often LID practices control up to the first 2 inches of runoff and therefore treat a much higher volume of water.

Are cost effective, saving developers the construction costs associated with subsurface collection systems associated with curbs and gutters. Installing curb and gutter/storm drain inlets and storm drainpipe systems are 2-3 times more expensive than engineered grass swales.